Ambient Air Measurement for Practical Users

testo 

Ambient Air Measurement for 

Practical Users 

testo 2 a edición 

testo 

testo 

0638.1445 

testo 

1st edition 

°C 

% rF 

td 

g/kg 

hPa 

m/s 

m 3 /h 

ppm 

CO 

ppm 

CO 2 

rpm 

mA 

mV

Foreword 

This handbook entitled "Ambient Air Measurement for Practical Users" was produced 

as the result of many discussions with our customers. It is intended primarily for air 

conditioning and ventilating plant acceptance and service technicians. The aim is not 

so much to show the experienced measurement technician when, why and what he 

should be measuring, but rather to identify the problems encountered in practice on 

location with temperature, humidity and air velocity measuring equipment. 

In this handbook you will find valuable hints on avoiding measuring errors on location; 

the best and most efficient way to use sensors, probes and measuring instruments 

and what marginal conditions you should take into account in order to interpret measurement 

results correctly. 

This handbook was produced under the overall control of our long-serving Product 

Manager and Project Innovation Manager, Manfred Streicher. Scientific knowledge 

was contributed by Professor Dr. Ing. S. Hesslinger. We should like to take this opportunity 

to thank them both very much for their valuable work, which Testo customers 

will be able to make use of. 

We would welcome further suggestions, which we would be pleased to incorporate 

into a new edition. If you have any questions or suggestions, please contact the Testo 

Ambient Air Measuring Engineering project team, the members of which are Product 

Manager Dipl.-Ing. (FH) Thomas Schwarzer and, with responsibility for innovation and 

project management, Dipl.-Ing. (FH), Physicist (bac) Manfred Streicher. 

The Management: Burkart Knospe 

Lothar Walleser 

3

Contents 

INTRODUCTION................................................................................................................6 

GENERAL PRINCIPLES OF MAKING MEASUREMENTS 

Measured Values in Ambient Air Engineering..............................................7 

Logging .....................................................................................................8 

TEMPERATURE MEASUREMENT 

Measuring Air Temperatures........................................................................12 

Measuring Errors When Measuring in the Unsteady State ............13 

Measuring Errors Due to Radiation .....................................................14 

Measuring Errors Due to Stratification ................................................15 

Surface Temperature Measurement ............................................................17 

Surface Temperature Measurements on Insulation 

and Poor Conductors (Wood, Glass, etc.).................................18 

Surface Temperature Measurement on Walls ......................................18 

Measurement on Pipes ........................................................................18 

HUMIDITY MEASUREMENT .................................................................................................. 

General ...................................................................................................20 

Measuring Relative Humidity in Rooms......................................................23 

Example ...............................................................................................25 

Measuring Relative and Absolute Humidity in Ducts ................................26 

AIR FLOW MEASUREMENT 

General ...................................................................................................29 

Planning Measuring Points..........................................................................33 

Air Flow Measurement in Ducts ..................................................................35 

Measuring Errors in Air Flow Measurement .........................................36 

Errors Due to Measurements at the Lower End of the 

Measuring Range.................................................................................37 

Errors Due to Sources of Disturbance..................................................39 

Obstruction of the Flow Section 

by the Probe ..............................................................................41 

Backflow Effects ........................................................................43 

Hints on Using Pitot Tubes ..........................................................................44 

Air Flow Measurements at Air Vents ..........................................................45 

Measurement at Suction Apertures .....................................................50 

Volume Flow Determination by the Fan Characteristic ............................52 

Measuring Room Air Velocity ......................................................................53 

4

PREPARING FOR AND CARRYING OUT MEASUREMENTS......................................................53 

PRESSURE MEASUREMENT ............................................................................................54 

CO 2 MEASUREMENT AS A MEANS OF ASSESSING ROOM AIR CONDITIONS..........................55 

GENERAL HINTS ON USING MEASURING INSTRUMENTS .....................................................57 

Using Probes and Sensors ..........................................................................58 

PRESENTING TESTO MEASURING INSTRUMENTS ...............................................................60 

Probe Specifications ....................................................................................63 

SOURCES AND BIBLIOGRAPHY........................................................................................68 

5

Introduction 

The 3 most important measured values in buildings or air conditioning and ventilating 

plants are temperature, relative humidity and air velocity. Essentially the requirement 

is to create a particular climate in a room, with temperature, humidity and air velocity 

controlled within a range of tolerance (e.g. the comfort range). At the same time, 

energy consumption to produce this result should be as low as possible. 

In any building there are four measuring locations with marginal conditions that are 

different in principle: 

6 

1. The external air, 

with extreme states: 

Winter (cold, high relative humidity, but low absolute 

humidity) and 

Summer (hot, high absolute humidity in some cases and 

average relative humidity as a rule) 

2. The plant room 

This houses the plant and 

equipment used to treat the 

external air drawn in: 

Drier, humidifier, filter, heating 

and cooling elements and fans 

and also the monitoring, control 

and adjusting units. 

Fig. 1: testo 454 in use in a plant room 

3. Ducting systems 

serving the room to be air conditioned. 

These comprise pipes (outward and return flow), air shafts (fresh air 

and ambient air), with branches and fixed measuring points. 

4. Contact with the room to be air conditioned: 

Air grilles, extractors and heat exchangers and radiators. 

. 5. The room itself, including walls, ceiling, doors and windows.

At all of these locations there are fixed measuring points with specified measured 

values (within permitted ranges of tolerance) for both acceptance and service 

purposes. The main marginal conditions are also indicated. 

If measurements are then made to confirm or check these measured values, the basic 

requirement is to obtain the same measured values at the same measuring points at 

different times but under the same marginal conditions. This calls for totally accurate 

measuring instruments which will produce the same measured values relatively independently 

of the marginal conditions. Otherwise, the measuring technician on location 

would have to ask himself every time what is the right measuring method, the correct 

sensor, and the best way to carry out the measurement (measurement duration, 

adjustment time, whether to take an average of several measuring points or an average 

over a period of time). 

To ensure measuring instruments are used correctly, it is important both to check the 

measuring equipment regularly and also to consider how best to use it on location. 

Ultimately, to be of value, measurement data need to be consistently and clearly 

recorded. A figure and a unit do not constitute a measured value. The measured 

value can only be interpreted and used as a representative measured value if it is 

accompanied by all the necessary information that makes it possible to repeat the 

measurement result under comparable conditions. 

General Principles of 

Making Measurements 

Measured Values in Ambient Air Engineering 

As part of the procedures for acceptance of a system, an expert checks whether the 

plant as built meets the contractual specifications. Any further measurements have to 

be agreed separately between the client (owner) and the contractor. 

At acceptance of an air conditioning plant, the measured values shown in Table 1 are 

determined as a rule. 

7

Table 1: Measured Values to be Determined in an Air Conditioning System as 

part of Functional Measurement 

Logging 

Logging the measurement results and marginal conditions 

forms part of every (acceptance) measurement. 

The data which have to be logged vary from one country 

to another. 

8 

Measurements Measurements 

Measured Values at the Plant in the 

Unit Room 

Fan motor current consumption 

X ----- 

Air flow 

Air temperature 

Humidity 

Prressure drop at filter 

Sound pressure level 

Ambient air velocity 

• Date/Time 

These are added directly to printouts made 

using Testo professional measuring equipment. 

It is advisable to do a short check printout 

immediately before measuring. 

• Name 

Often, despite taking all due care, not all of 

the marginal conditions are noted down. 

The name of the person making the measurement 

should therefore be recorded 

for any queries. 

X X 

X X 

X X 

X ----- 

----- X 

----- X

• Equipment Specification 

For subsequent maintenance and servicing it has to be ensured 

that important plant specifications have not changed since the last 

measurement (e.g. more powerful fan). 

• Plant Operation 

This covers data in respect of desired values set, plant operation under 

full load, etc. 

• Measuring point 

It must be possible to assign the measured values to the measuring point 

accurately (e.g. on a drawing of the plant). 

• Measuring Equipment 

Details of the measuring equipment used and the sensor. The marginal 

parameters set on the measuring instrument may also be included, e.g. 

absolute pressure at the measuring point, atmospheric density set or 

details of other parameters used to compensate for measured values 

in the measuring equipment (temperature, humidity, etc.) 

• Measuring Range Set 

This is important in the case of measuring equipment with a choice of measuring 

ranges, because a different accuracy is assigned to each measuring 

range. For Testo measuring instruments it is not relevant, because they 

automatically select the appropriate range when the correct sensor for the 

purpose is selected and fitted. 

• Desired Value 

The value that should be obtained under specified conditions. 

• Actual Value 

The measured value read off or printed out. 

• Variance 

• Notes on the Measuring point 

Note down everything that could have affected the measurement, such 

as maintenance work, measurements before or after changing filters, 

velocity measurements with vent grilles removed, doors and windows 

open or closed, etc. 

• Weather Conditions 

Barometric pressure, wind velocity, wind direction or external air 

temperature and humidity. 

9

Fig. 2: Example of a correct log of an air flow measurement using the 

centroidal axis method 

Further examples on the next page are a printout of a log made on location with 

measuring equipment and a diagram produced on the PC to document the behaviour 

of the temperature of a radiator over time. 

10

Fig. 3: Documentation using Testo testo 454 printing format 

Comfort software 0554.0171 in conjunction with 0554.0345 

infrared printer 

If the measuring point is not clearly shown on the drawings, a sketch should be made 

of the measurement environment and the measuring points marked on it with dimensions. 

Dimensions should be taken from the fixed points of the building (e.g. pillars). 

5000 

3000 

Graphic Printout 

Conditions: Outward / Return Flow Temperature 

Notes Building: Maienstraße 8 

Maier, Mr. Armbruster 

Room 3, radiator below central window 

Valve closed and room ventilated before 

start of measurement 

TITLE: Sketch for Measurement Log 2 DATE: ........................... 

MEASURING POINT: Office 2 NAME:........................... 

Desk 

Measuring 

Point 

x 

Window 

Fresh Air 

Exhaust Air 

Title:10_ROOM 

GROUND PLAN 

2000 

Door 

2000 

4000 

4000 

Fig. 4 is an example of a sketch showing the measuring points for 

temperature stratification measurement in a room. 

Page 

ROOM 

ROOM 

3000 

Desk 

Room 

Air Vents 

MP 3 

MP 2 

MP 1 

100 

SECTION 

1300 

1800 

11

Temperature Measurement 

Measuring Air Temperatures 

Temperature measurements are made as part of acceptance measurements at the 

central unit of the air conditioning plant, in the fresh air and in the air conditioned 

rooms. 

New requirements have been laid down for room temperatures in connection with 

comfortable temperatures for human beings. 

Limits apply to the following temperatures: 

12 

• Ambient Air Temperature 

Ambient air temperature should be measured with a radiation-protected 

thermometer. All Testo temperature probes with stainless steel tubes are 

suitable for the purpose. In Germany, for example, the vertical temperature 

gradient of the ambient air must not exceed 2 K/m. 

• Operating Temperature (or sensation temperature) 

This is measured using a globe thermometer. According to Glück [4], when 

using a globe thermometer with a diameter of 150 mm (see Testo globe 

thermometer probes), the measured operating temperature coincides with 

the human sensation temperature at ± 0.41 K. 

Calculating the operating temperature from the measured values for ambient air 

temperature and the surface temperature of the surrounding surfaces of the room, 

weighted by the insolation factors, involves a great deal of time and calculation and 

will therefore not be discussed in greater detail here. Moreover, using the Testo 

globe thermometer means that this method no longer need be used. 

When using globe thermometers it is important to ensure that the globe is not exposed 

to direct sun or light rays, because only the radiation temperatures of the surrounding 

surfaces should be taken into account in the temperature measured, i.e. either select 

the measuring point accordingly or, if this is not possible, shade the globe at as great 

a distance as possible. Allowance should be made for the inertia of the thermometer 

by measuring over a suitably long period (until a constant value is displayed - at least 

20 to 30 minutes).

Temperature measurements in the room (air temperature and operating temperature) 

should be made at the same point and under identical marginal conditions, as should 

ambient air velocity measurements. 

Measuring Errors When Measuring in the Unsteady State 

Measuring and recording marches of temperature requires temperature probes to be 

used, the time constant of which needs to be matched to the rate of change of the 

temperature of the medium. 

The time constant depends on heat transfer to the probe surface, the thermal conductivity 

of the probe material and the heat-storage properties (mass, density, thermal 

capacity) of the probe. 

Probe temperature 

Temperature of the medium 

➀ Temperature probe 0600.9794 (NiCr-Ni) 

② Temperature probe 0610.9714 (NTC) 

100 % 

63% 

T 1 

T 2 

1 

2 

(T probe time constant) 

Time 

Fig. 5: 

Step responses of two 

temperature probes with 

different time constants 

(T2 = 2T1) 

The type of probe to be used depends on the application. We recommend 

probe ➁ in situations where the conditions change slowly but a high level of 

system accuracy is called for (measurements in rooms, comfort, system 

accuracy > 0.5°C). 

13

Probe ➀ should be chosen for measurements in the duct (e.g. when rapid temperature 

changes take place following a sudden change in the proportion of ambient air). 

Fig. 6 shows in quality terms the temperature difference between the temperature of 

the medium and the temperature display. The temperature of the medium is assumed 

to rise at a constant rate. 

Fig. 6: 

Illustration of reading 

error at a constantly 

rising medium temperatu- 

Temperature difference 

14 

T 

Temperature of the medium 

Probe temperature 

( T Probe time constant) 

Reading 

error 

re due to the inertia of the 

temperature probe 

A slow probe (no. 2 in Fig. 5) leads to measuring errors in dynamic processes despite 

being highly accurate at equilibrium. The faster the probe, the more realistic is the 

measured value displayed. 

The probe t 99 time should be at least twice as fast as the expected change in the 

temperature of the medium. 

Measuring Errors due to the Influence of Radiation 

The heat exchange due to radiation between the temperature probe and the surrounding 

surfaces should be taken into account to ensure accuracy of air temperature 

measurements. Considerable deviations from the actual air temperature can occur 

because of the temperature difference between the probe and the surface and the 

distance between them. 

Time

Fig. 7 shows an air temperature measurement using a thermometer not protected 

against radiation behind an air heater. The measuring error (too high temperature) 

would be even greater with a low convective heat transfer (due to low air velocities). 

Air 

Heater 

Surface temp. = 50 ° C 

Air temperature = 20 ° C 

Probe temperature = 21 ° C 

Probe 

10 cm 

Fig. 7: 

Air temperature measurement 

using a thermometer 

not protected 

against radiation 

In this example, the temperature measurement using a probe without radiation protection 

produces a measuring error of 1 K. 

In practice, avoid measuring air temperatures in the immediate vicinity 

of surfaces the temperature of which differs greatly from the air temperature. 

If this cannot be avoided, screen the probe from incident radiation 

by means for example of a metallic bright aluminium sheet between the 

radiation source and the probe. 

Measuring Errors due to Stratification 

When measuring air temperature in ducts and at the central unit, in many cases only 

single-point measurements are carried out. Such measurements are only representative 

of the total air flow if the mean temperature and mean velocity of flow of the section 

are recorded at the measuring point. Fig. 8 shows an example where these conditions 

cannot be met. 

15

16 

a) Qualitative mixing sequence at V um/V a = 1/1 

b) Characteristic velocity profile at measuring plane III 

c) Temperature gradient over the height of the chamber 

Fig. 8: 

Mixing chamber for air 

with temperature stratification 

and backflow areas 

Temperature stratification can occur if air flows at different temperatures are mixed or 

after passing through heat exchangers, especially if the surface temperature varies 

greatly across the exchanger section (often in case of part load). 

If an almost completely mixed flow is not available at the measuring point (comparable 

with air flow measurement), a grid measurement should be made (see also page 31). 

The number of measuring points is determined by the irregularity of the temperature 

profile. 

The temperatures measured have to be weighted by the associated velocities in order 

to determine the mean temperature of the total air flow accurately.

Measuring Surface Temperatures 

The primary reason for measuring surface temperatures is to establish marginal conditions 

accurately. This applies to possible pronouncements as to the operation of 

heat exchangers, either for air heating or cooling in ducts or on radiators in rooms. 

Fig. 9: 

Measuring the surface 

temperature of a radiator 

using magnetic probe 

0600.4893 

Surface temperatures also play a part in thermal balances or in the evaluation of 

systems (e.g. air ducts carrying hot air or pipes containing coolant). 

Another area where surface temperatures play an important role is that of comfort, 

where the influence of the surfaces enclosing the room, such as windows or walls, 

has to be assessed. 

Condensation phenomena depend on surface temperatures, particularly in the case 

of relative humidity measurements (see Fig. 14, page 25). 

The condition of large motors, fans or pumps can be more accurately gauged by 

measuring the surface temperature of the housing or bearings. 

The easiest measurements to make are those on large metal parts (large mass) 

with a flat, smooth surface. The good thermal conductivity of metals guarantees 

efficient heat transport from the core of the surface to be measured to the actual 

measuring probe. The heat transmission from the surface to the probe (the critical 

factor for accurate measurement) can be markedly improved by using heat paste 

(part no. 0554.0004). 

17

Another source of error is reading off the value too soon without allowing for the 

probe temperature adjustment time and the temperature drop on the surface caused 

by contact with the temperature probe. This varies according to the model. 

18 

Tip for practical measurements: 

place the probe in position vertically and 

wait for the measured value to stabilise 

(temperature adjustment time). 

When making surface measurements, 

the maximum measured value displayed 

is closest to the true measured value 

( → maximum value display on the 

measuring instrument). 

;;; 

;;; 

Surface measurements on insulation and poor conductors 

(foam, wood, glass, etc.): 

In this case, stick-on thermocouples the surface of which can be connected to the surface 

via heat paste are the best type. The low mass of these sensors means as little 

heat as possible is drawn out of the surface, while virtually no heat is removed via the 

connecting wires. The ideal method is to attach the sensor to the surface with heat 

paste and fasten it in place with generous quantities of insulating tape, including part 

of the connecting cable as well. 

Wall surface temperature measurement: 

The best instrument for this application is the contactless infrared surface temperature 

probe, particularly because they measure integrally over a large area of the wall at a 

corresponding distance, i.e. a mean across these areas is produced automatically. 

Their practical use is rather problematic, however, because the potential error source 

(incorrect allowance for the emission level) is difficult to control. Experience values 

and comparative measures with contact thermometers can help overcome this, however. 

Measurements on pipes 

Flat surface probes should not be used to measure curved surfaces. Look for a flat 

measuring surface (flange, union nuts) or use the specially designed pipe surface probes. 

These ensure the sensor is correctly positioned,

particularly when making measurement over a longer period. 

Fig. 

10: testo 454 in use 

with pipe surface 

probe 0600.4593 

Important when making differential measurements: 

Because of the probe tolerances, 2 probes placed immediately next to 

one another on a pipe will not necessarily show a differential temperature of 

0 °C. Carry out this test before trying to measure differential temperature 

using different probes on pipes of different temperatures. In case of 

devia tions, a subsequent correction should be made to the measurement 

result if necessary. 

To sum up, correct temperature measurements are made as follows: 

Any measuring instrument measures the sensor temperature directly, and the temperature 

of the medium or object only indirectly. Testo optimises the transition from one 

application to another through different probe designs. 

The user should select the appropriate type of probe and use it correctly: 

e. g. • Measurement on flat metal surfaces - rugged - 0600.9993 

• Measurement on curved surfaces - fast - 0600.0194 

• Measurement on pipes - self-attaching probe - 0600.4593 

• Long-term measurements on radiators - magnetic probe - 0600.4793 

• Measurement on surfaces which are poor heat conductors - 

stick-on thermocouple- 0644.1607 

19

Humidity Measurement 

General 

Controlling atmospheric humidity is important wherever people or humidity-sensitive 

substances are in a room for a comparatively long period. A humidity exchange takes 

place between the ambient air and the people or substances in the rooms. 

Hygroscopic substances release humidity into the environment or absorb moisture 

from it. In doing so, they are seeking a balance between their humidity content and 

the environment. This means that where the air has a low relative humidity content, 

hygroscopic substances dry out slowly; if the relative humidity is high, materials 

enrich themselves with water. 

To give an example: in low atmospheric humidity, materials become brittle and crack 

(especially when heating is on during the winter). At high relative atmospheric humidity 

and partly cool surfaces, materials start to swell. Condensation forms, and in some 

cases mould fungus may grow. 

The driving force behind this is the degree of relative atmospheric humidity. An optimum 

relative humidity range can be defined for people and hygroscopic materials. 

In the case of human beings, this is called the comfort range. In buildings, this can 

also be transferred to other things such as wood, materials, wallpaper, paper, etc. 

As long as the condition of the air remains within this range, it may be assumed that 

excess moisture transport will not take place and there will be no essential change to 

the properties of substances or materials. 

20

Tempperature° C 

Fig. 11: Comfort range 

Standard 

atmosphere 

The relative humidity value always refers to a temperature. This is due to the 

definition of relative humidity: 

Relative humidity is calculated from the existing (absolute) humidity in relation 

to the maximum possible (absolute) humidity at the prevailing temperature. 

When the maximum possible humidity is reached, this is referred to as the 

temperature of dew point and the relative humidity is 100% RH: condensation 

water or mist forms, and the air is saturated with moisture. 

As a rule, humidity measuring instruments are specified with 

an accuracy of ± 2% RH. In most cases, however, the values are 

only valid for a narrow temperature range (18 - 25 °C). 

Testo compensates for this dependence by means of an 

additional temperature measurement. 

21

Testo humidity probes are generally fitted with humidity and temperature sensors. 

This gives you the two corresponding values, temperature and humidity, at a glance. 

The temperature of dew point calculated from these values can also be displayed if 

required. 

Capacitive humidity probes to measure relative atmospheric humidity are generally 

maintenance-free. It is recommended that they be checked with a testing and calibration 

set from time to time. 

Velocity of flow also plays a part in surface moisture exchange. As the air speed 

increases, the processes are accelerated, because a local equilibrium, a limit air layer 

in the vicinity of the surface, cannot be reached. 

22

Measuring Relative Humidity in Rooms 

Essentially, the same rules apply as for temperature measurement. At constant 

absolute humidity, relative humidity is a function of temperature. There is a risk of 

stratification and sizeable changes in humidity in the vicinity of surfaces the 

temperature of which differs greatly from the air temperature. 

The minimum distance from the wall is reached when despite altering the distance, 

the humidity and temperature values displayed no longer change. 

; 

; 

; 

; 

; 

; 

; 

; 

70 % RH 

T OF = 15 ° C 

T Room = 25 ° C 

50 % RH 

Minimum distance wall/measuring point for representative 

atmospheric environment measurement 

Two main types of measurement are made in the room: 

Fig. 12: 

Relative humidity in 

rooms 

• the spot check measurement: 

A rapid check measurement is most useful if a strong or sudden exchange 

of air takes place, i.e. in the case of air conditioning plants with a large 

change of air, frequent opening of windows or doors, major humidity 

sources in the room itself, as in the case of kitchens and bathrooms, etc. 

Make sure the humidity probe takes the same temperature as the air to 

be measured. Move the probe around if you are measuring in 

stagnant air (the adjustment times can be shortened). 

Avoid exposure to the direct rays of the sun. Even a few tenths of a 

degree of a temperature difference between the probe tube and the 

ambient air can distort the measurement result. 

23

24 

• the long-term measurement for observation 

The best way to investigate the effects of humidity in the vicinity of surfaces 

is with data storage equipment over a long period, because more than one 

influence may occur simultaneously (effect of the air conditioning plant, wall 

temperature, periodic fluctuations: day and night, lower temperature at 

weekends, etc.) 

Fig. 13: 

Long-term 

measurement 

with the testo 

171-2 Data Logger 

This produces data which can be interpreted from a number of points of view, and 

can also be used in complex processes.

Illustrative example of a boiling kettle in a kitchen: 

In this simple yet very illustrative example, the air temperature remains constant 

at just over 20 °C even after the hotplate has been switched on, as does the 

somewhat cooler metal surface of the sink. The surface temperature of the window 

rises slightly after condensation starts to form. The absolute atmospheric humidity 

(dew point) rises sharply as the water boils. Condensation forms on the window 

after only a short time. 

No condensation forms on the sink because its surface temperature is higher than the 

temperature of dew point, even at the maximum of absolute humidity. 

Conditions 

Notes 


Sink temperature 

Window 

temperature 

Graphic Printout 

Steam Formation in a Kitchen 

Name: 

Streicher on 4.3.1995 

Illustrated: Development of 

absolute humidity after the 

kettle is switched on. 

Atmospheric humidity, dew point 

Title: 2_KITCHEN 

KITCHEN 

KITCHEN 

KITCHEN 

KITCHEN 

Fig. 14: Diagram as a log; illustration of development through time 

Page 

25

Measuring Relative and Absolute Humidity in Ducts 

The prime consideration when measuring humidity in ducts is the absolute humidity. 

Measurements are made upstream and downstream of components relevant to humidity 

(drier, humidifier) or at points in the system where air flows of different absolute 

humidities are mixed (ambient air / external air). 

The absolute humidity measured may be assumed to remain constant over long stretches 

of duct, provided there is no additional drying or humidifying en route (or condensation 

formation on cold surfaces). 

One simple method for roughly estimating the conditions starts from the assumption 

that the air directly behind a washer may be considered saturated with moisture. The 

temperature is measured at this point. This temperature may be assumed to be approximately 

equivalent to the temperature of dew point. This applies analogously to 

efficient condensate traps. 

If the temperature in the air conditioned room is then measured, the relative humidity 

in situ can be calculated from tables. 

In practice, however, only humidity measurement using a humidity meter provides 

usable values which can then be used in calculations, because measurements also 

have to be made at humidifiers with a level of humidity < 100% (e.g. only 80% RH 

downstream of the humidifier). 

The basis for drawing up humidity balances with the associated energy consumption 

picture is the MOLLIER diagram. This diagram is used in practice because working 

with formulae on location is awkward. The MOLLIER diagram consists of superimposed 

families of curves, each curve representing a constant value. 

There are thus straight lines for constant temperature ( ϑ ), lines for constant relative 

humidity (ϕ), lines for constant absolute humidity ( x ) and lines for constant caloric 

content or enthalpy ( h ). 

26

const 

const 

const 

const 

satuvation 

Fig. 15: 

Structural principle of a MOLLIER diagram 

The state of the moist air is clearly defined in terms 

of two of these values. The other two parameters 

may be read from the diagram, i.e. with a humidity 

and temperature measurement you can allocate a 

point on the diagram to the air at the measurement 

point. 

All technical changes (heating, cooling, drying, 

humidifying) to this air correspond to a movement 

within the diagram. 

Structure of a MOLLIER diagram 

h Caloric content kJ/kg dry air 

x Water content g/kg dry air 

ϑ Air temperature °C 

ϕ Relative atmospheric humidity %RH 

Hints for practical measurement: 

Do not use electronic humidity probes to measure dew points higher 

than the probe temperature, because the ensuing condensation 

- saturates the sensory mechanism for a certain time, or even 

- condensation forms on the electronics, impairing the function 

of the probe (until it dries out). 

For some simple examples, see overleaf the changes of state of the moist 

air caused by an air heater or air washer. 

27

Fig.16: Change of air state in Fig. 17: Change of air state in 

an air heater an air washer 

e State of air on entry e State of air on entry 

a State of air on exit, at (e.g. prewarmed external/ 

∆ h = 15 kJ/kg heat supply ambient air mixture) 

a State of air on exit for 

If e and a are swapped, the diagram humidity µ=0.76. 

is also valid for air coolers with a 

dry surface 

The air heater is characterised by heat supply at constant absolute humidity. This 

leads to an increase in temperature in conjunction with a drop in relative atmospheric 

humidity. 

In the air washer, water is added to the air at constant caloric content. The addition 

of water in conjunction with the cooling of the moistened air increases the relative 

humidity. 

This illustration using air will help you to understand the steps in the diagram from 

the point of view of air conditioning and gives you the relevant parameters at a glance 

to determine humidity, temperature and the energy balance in air conditioning plants. 

This gives you the necessary energy consumption for every change of state of the air. 

28

Air Flow Measurement 

General 

Keeping air flows at the desired level is crucial to the operation of the plant. If air 

flows are smaller than planned, the loads from the room (heating, cooling and 

substance loads) may in some cases not be removed. This makes measuring air 

flows as accurately as possible very important. 

Choose the right probe to suit local conditions before making the measurement. 

Suitable probes (according to type) for the following applications are: 

→ low velocities of flow: 

thermal probes 

→ medium velocities of flow, particularly in ducts: 

vane probes with the smallest possible diameters, 

→ measurements at extractor grilles and vents: 

large-diameter vane probes 

(60 or 100 mm) 

or 

→ measurements at high velocities, 

in heavily contaminated flows with a high proportion of particles: 

Pitot tubes 

29

100 m/s 

60 m/s 

40 m/s 

20 m/s 

10 m/s 

30 

Velocity of Flow up to + 500° C 

Pitot tube 0635.2245 

Ø 4 mm, l = 300 mm 

(not shown) 

up to + 350° C 

Pitot tube 

up to +140 ° C 

0635.9540 

Ø 16 mm 

up to + 60 ° C 

0635.9449 

Ø 60 mm 

0635.9349 

Ø 100 mm 

up to + 50 ° C 

0635.1042 

Ø 10 mm 

up to + 140 ° C 

0635.9640 

Ø 25 mm 

up to + 100 ° C 

0635.9443 

Ø 12 mm 

up to + 70 ° C 

0635.1549 

Ø 4 mm 

0635.1049 

(with telescope) 

Ø 4 mm 

0635.2045 

Ø 7 mm, 

l = 500 mm 

(not shown) 

up to + 350 ° C 

0635.6045 

Ø 25 mm 

Fig. 18: 

Choose the best 

velocity probe for 

your application 

0635.2145 

Ø 7 mm, 

l = 350 mm 

(not shown) 

In conjunction 

with a pressure 

probe in each 

case

When making acceptance measurements, indirect measuring methods (grid 

measurements) are used to determine air flows. 

The following methods are suggested: 

• Trivial method for making grid measurements in rectangular sections 

With this method, no particular assumptions are made regarding the velocity 

profile. The velocity field inside the rectangular duct section is divided into 

measuring areas of equal size, with the actual measuring point lying in the 

centre of each measuring area. 

Fig. 19: 

Grid measurement diagram 

With a constant velocity profile, only a few measuring points yield a 

representative measuring result. 

However, if sizeable differences of flow velocity are noted across the 

section, the number of measuring points should be increased. The 

number of measuring points is sufficient if the measured value of any 

area is representative of its immediate vicinity, i.e. if it can be regarded 

as a true mean for its part area. 

• Centroidal axis method for grid measurements in circular sections 1 

With this method, the circular duct section is divided into annuli of equal 

area, the measuring point lying on the centroidal axis of the annulus 

(not on its centre line). 

Velocity v 

Fig. 20: 

Centroidal axis method 

Here again, the measurement is evaluated 

by taking the arithmetic mean of the individual 

measured values. 

31

32 

• Log-linear method for making grid measurements in circular sections 

This method is used where there is a turbulent limit layer profile, something 

only rarely encountered in practice. In this method, the measuring points are 

distributed over at least 2 diameters perpendicular to each other, the 

distance from the measuring points to the edge decreasing logarithmically. 

Fig. 21: 

Log-linear method 

The mean velocity of flow can be calculated from the individual velocity measured 

values and from that, the air volume flow. 

•V = volume flow in m 3 

/h 

v = mean velocity of flow in m/s 

A = flow section in m 2 

Example: For a section A of 0.5 m 2 

and a measured mean velocity of 4 m/s, 

the volume flow is 7,200 m 3 

/h 

Fig. 22 shows ideal flow profiles in the duct. On the left is a marked laminar flow with 

a clear maximum at the centre of the duct. The mean velocity of flow is around 

one-third of the duct diameter. On the right is a turbulent limit layer profile (no swirls) 

with a largely constant flow at the centre of the duct and a drastic drop at the walls. 

Between these two ideal forms, any intermediate forms can occur, making grid 

measurement essential as a rule.

laminar 

Fig. 22: 

Ideal velocity profiles 

in the duct 

Air flows are often measured at individual air vents as part of adjustment work. Since 

such measurements take a lot of time and are also associated with considerable 

errors (see pages 45 onwards), it is advisable to make the adjustment on the basis of 

the results of the duct grid calculation and then check that the vents are equally exposed 

(for instance by carrying out smoke tests). In this case, only the main air flows 

need be measured. Appropriate measuring points should be selected, and this in turn 

calls for proper measuring point planning. 

Measuring Point Planning 

turbulent 

The most important condition for accurate measurement is the correct choice of measuring 

point. This is particularly true when making air flow measurements in ducts. 

Accordingly, the plant designer should mark the measuring points on the execution 

drawing (project drawing). The following criteria apply: 

• Air flow measuring points should be incorporated into all the main ducts 

and on the supply pipes to the rooms with strict requirements. 

• Minimum distances from sources of disturbance should be observed 

(see also the section entitled Errors due to Sources of Disturbance on 

page 39). A distance of not less than 6 times the hydraulic diameter 

D h = 4A/U (A = duct section, U = duct circumference) should be observed 

from upstream points of disturbance. A distance of 2 D h from downstream 

points of disturbance is sufficient. 

33

34 

Orifice plates Valves 

Zero 

Elbow 

Fig. 23: 

Problematic measuring 

points 

Hints for practical measurements: 

These measuring points cause problems because the velocity 

profile can change sharply in their vicinity. The maximum flow 

is no longer at the centre of the duct: areas with no velocity of 

flow and areas of backflow can occur. 

• The measuring point must be accessible. 

• There must be enough room to use the measuring instrument. 

• It should be checked that the duct was constructed as planned, including 

the measuring points. 

• The measuring points should be identically 

marked on the drawings and on the plant itself 

(see also notes on logging, page 8 onwards). 

The marking should include: 

- plant identification (e.g. conference 

room, 1st floor) 

- measuring point (e.g. fresh air) 

- desired and actual volume flow 

- measuring instrument and measuring 

principle (e.g. vane anemometer) 

- date 

- name of company or person responsible 

for measurement 

Max

Air Flow Measurement in Ducts 

Before starting to make measurements, the logs should be prepared (see 

also "Logging", page 8 onwards), taking the desired values from the inspection 

drawings, for example. The plant operating mode should be checked and noted 

in the log. It must be ensured that the operating mode does not alter during 

measuring (e.g. due to adjustments). 

Measurements should start at the central unit of the air conditioning plant in order 

to avoid unnecessary measurements on individual branches if the total air quantity 

in the main duct is insufficient. 

On completion of each measurement, the measurement results should be evaluated 

immediately. The best measuring instruments are those which print out the individual 

measured value and the arithmetic mean calculated. 

Log 

FRESH AIR 

Fig. 24: 

testo 454 printout 

File this printout with the measurement log. Be sure to enter the main marginal 

parameters (in this case: company, name of person responsible, date, measuring 

point, measuring method, result of measurement, deviation from desired value, air 

temperature and - in the case of Pitot tube measurement - the air density used in t 

he calculation and associated absolute pressure and relative humidity). 

35

Measuring Errors in Air Flow Measurement 

A distinction is made in the theory of measurement engineering between random and 

systematic errors. 

Random errors can be computed by error calculations. Uncertainties due to the set-up 

of measuring instruments, measuring method, measuring equipment and readings are 

combined to give a total uncertainty. 

The figure below shows an example of an error calculation. 

Measuring section a • b in mm 2 

Uncertainty of meas. point τU in % 

Number of meas. points n Uncertainty of anemometer τG in % v.E. 

Relative distance from Uncertainty of reading 

source of disturbance a/Dh (where display changes) δv in m/s 

Velocity = reading value v in m/s Anemometer meas. range vE in m/s 

Irregularity of profile U in % Uncertainty of side length δa, δb in mm 

Uncertainty: 

Fig. 25: Example of an error calculation 

δ a 

2 

τL = ± τ 2 

U + 100 + 100 2 

+ 100 2 

+ τG 2 

a b v v 

τL = ± 9 2 + 100 2 

+ 100 2 

+ 100 2 

+ 1 2 

For a = 1200 mm, b = 800 mm, n = 20, a/Dh = 3, v = 6 m/s, U = 24 % (see Fig. 28), τU = 9 % 

(see Table 2), τG = ± 1 % v. E. (Table 2) δv = 0,2 m/s, vE = 40 m/s, δa = δb = 1,5 mm 

the following is true: 

1,5 1,5 0,2 40 

1200 800 6 6 

36 

= ± 11,69 % 

δ b 

v = 1,2 m • 0,8 m• (6,0 ± 0,7) m/s = (5,76 ± 0,7) m3/s = 20880 ± 2520 m3/h Result v = 6m/s 

Uncertainty τ = ±11,69 % = ± 0,7 m/s 

Therefore the result for the volume flow is: 

δ v 

v E

It is acknowledged that the errors due to the use of the measuring instrument at the 

lower end of the measuring range and the irregularity (distortion) of the flow profile 

are particularly critical to the overall error. These major sources of error will therefore 

be discussed in greater detail later. 

Besides random errors, there are also what are called systematic errors. These are 

"hidden" errors which occur when the person making the measurement is unaware 

of various factors which can distort the measurement result. 

A good example is the measurement of air velocity in the backflow area of a curve 

using a measuring instrument which displays positive values irrespective of the direction 

of flow (see section on Backflow Effects, page 43). 

Errors due to measurements at the lower end of the measuring range 

Depending on the measuring method, flow sensors show a varying error characteristic 

which can adversely affect the measurement result, according to the measuring range 

selected. 

For example, thermal velocity probes have a very small intrinsic error (in the ± 2-5 

cm/s range), to which has to be added a sensitivity error as a percentage of the 

measured value (2.5 - 5% of the measured value). This means that thermal probes 

are mainly suitable for the measurement of low air velocities, as the uncertainty of 

measurement increases linearly as the air velocity rises. 

In the case of vane probes, in contrast, the uncertainty of measurement is normally 

given as a percentage of the final value. Vane probes thus have a constant mea 

suring error right across their measuring range. As a result, vane probes are most 

suitable for use in the top half of the measuring range. It can be said as a rule that 

thermal anemometers are preferable up to 5 m/s and vane probes over 5 m/s. 

37

Fig. 26: Measuring errors due to the use of a vane probe (dia. 16 mm) in 

conjunction with a testo 452 at the lower end of the measuring range 

The measuring point has a very uniform flow profile (distance from the "fan" source of 

disturbance approximately 12 D h), and the inaccuracy of measurement is therefore 

essentially attributable to the use of the vane probe at the lower end of the measuring 

range. In measurement series 2, with a higher air velocity (4 m/s), the uncertainty of 

measurement drops from to 19.5% down to 8.2%. 

The error characteristic is even better with Pitot tube measurement at average and 

high air velocities. Due to root calculation when computing the air velocity from the 

dynamic pressure, the measuring error decreases sharply as the air velocity increases 

(despite a constant intrinsic error in the pressure measurement). 

38 

SECTION A-A 

57 170 

59 

178 

296 

415 

Meas.Values (m/s) and Results of Meas. Series 1 

Reference Volume Flow 478 m 3 /h 

Dimension 

57 170 Mean velocity 

1.5 m/s 

Duct section 

0.106 m 2 

Volume flow 

571 m 3 

59 1.4 1,5 

178 1.5 1,6 

296 1.5 1,5 

/h 

Deviation from 

415 1.5 1,5 

reference value 

+19,5 % 

A 

A 

MEASURING PLANE 

3700 

RECTANGULAR DUCT 225 X 475 

FAN 

AIR 

INLET NOZZLE 

Meas.Values (m/s) and Results of Meas. Series 2 


57 170 Mean velocity 

4.3 m/s 

Duct section 

0.106 m 2 

Volume flow 

1637 m 3 

Dimension 

59 3.9 3.9 

178 4.2 4.2 

296 4.5 4.5 

/h 


415 4.3 4.3 


+8,2 %

Errors due to Sources of Disturbance 

The measurement of air flows (air velocities and dynamic pressures) requires a flow 

which is free from swirl and backflow. The measuring point selected should therefore 

be sufficiently far from the source of disturbance. A swirl flow is stable and can continue 

over long distances in the duct system. In such cases, a flow rectifier should be 

fitted upstream of the measuring point. 

Backflows which may be caused by flaps and elbows or knees generally cease 

at a distance of twice the hydraulic diameter (2 D h; D h = 4 x A/U) from the source 

of disturbance. However, the flow profile is so distorted that a large number of 

measuring points are necessary in order to keep the uncertainty of measurement 

low. Fig. 27 shows flow profiles measured with a Prandtl Pitot tube after a bend 

for 3 different distances from the disturbance source. 

10 m/s 

5 m/s 

Distance 

7 x D h 

Balanced flow profile Distorted flow profile 

10 m/s 

5 m/s 

10 m/s 

Distance 

2 x D h 

5 m/s 

Distance 

1 x D h 

Flow profile 

with backflow 

Fig. 27: 

Decline in the irregularity 

of the flow profile as 

the distance from the 

disturbance source 

increases 

The horizontal 

velocity profiles 

were measured with 

a Prandtl Pitot tube. 

The number of measuring points to be selected in a section will be found in Table 2. 

The irregularity (distortion) of the flow profile should first be determined form Fig. 28. 

D=250 

Air 

39

Relative distance a/Dh of the measuring 

point from the disturbance source 

40 

10 

8 

7 

6 

5 

4 

3 

2 

Number of 

Measuring 

Points 

4 

5 

6 

8 

10 

20 

30 

50 

100 

200 

1 

4 5 6 7 8 10 20 30 40 % 

Irregularity of profile U 

Uncertainty of Measurement τU in % 

Irregularity of Profile U in % 

2 10 20 30 40 

6 12 20 28 36 

5 11 17 24 31 

5 10 15 21 27 

4 8 13 18 23 

3 7 12 16 20 

2 5 8 11 14 

2 4 7 9 11 

1 3 5 7 8 

1 2 3 5 6 

1 1 2 3 4 

Fig. 28: 

Empirical interrelationship 

between the irregularity 

of the flow profile and the 

relative distance a/D h from 

the disturbance source 

a = distance from disturban 

ce source 

D h = hydraulic diameter 

Table 2 : 

Uncertainty of measurement 

with approximately 

swirl-free flow as a function 

of the number of 

measuring points 

For example, a measuring point at a distance of 2 D h from the disturbance source 

would have a profile irregularity of around 40%. If the uncertainty at the measuring 

point is to be kept below 10%, more than 40 measuring points are necessary. 

From these examples you can see that the measuring errors in flow measurement are 

primarily due to the conditions at the measuring point, the choice of the correct probe 

and correct use. Instrument tolerances, on the other hand, may be disregarded. 

50 

42 

36 

32 

27 

24 

16 

14 

10 

7 

5

Measurements with Vane Anemometers 

• Obstruction of the flow section by the probe 

The ideal vane probe for duct measurements is the 16 mm diameter combined 

flow/temperature probe. This probe may be regarded as a universal probe, because 

on the one hand the vane section is big enough for response effects and dirt from 

storage not to have too great an influence, while on the other hand the dimensions 

are small enough for the test aperture diameter to be within the bounds of possibility. 

This probe is particularly suitable for measurements in large air ducts in conjunction 

with a telescope. When making measurements in small duct sections, the influence 

of the vane section on the accuracy of measurement should not be disregarded, 

since it increases as the duct section decreases. 

The flow velocity measured increases as a function of the depth of penetration of 

the vane compared to the flow velocity upstream of the probe. This effect is caused 

by the restriction of the section of the clear flow section when the probe is inserted 

into the duct. 

Fig. 29: 

Restriction of section 

A1: Probe section in 

the flow path 

A2: Clear 

duct section 

• For an illustration of this effect, see the comparative measurement in Fig. 30 

between a 25 mm diameter vane probe in comparison with a Prandtl Pitot tube 

with an appreciably smaller section on a 250 mm diameter duct. 

41

42 

y = 0 

A 

A 

D=250 

x = 0 

Section A-A 

Distance = 7 x D 

Measured Values (m/s) and Results of Meas. Series 

with an Electronic Vane Probe (dia. 25 mm) 


Dimension 

8 

26 

49 

81 

169 

202 

224 

242 

y x 

7..8 

9.5 

10.1 

10.7 

11.9 

12.5 

12.7 

12.0 

8.2 

9.6 

10.1 

10.9 

12.2 

12.5 

12.8 

11.8 

Reference volume flow= 1512 m 3 /h 

Mean velocity 

11,0 m/s 

Duct section 

0,0491 m 2 

Volume flow 

1937 m 3 

/h 



+28 % 

Fig. 30: Measuring errors due to the restriction of the clear flow section by a vane 

anemometer in comparison to Pitot tube measurements. 

The reference volume flow was determined by the inlet nozzle. 

• The measured values in the series of measurements with the vane anemometer 

increase with the depth of penetration (e.g. from 9.5 to 12.7 m/s in the vertical 

plane). 

Because of the section restriction by the vane anemometer, the mean of all the 

measuring points shows a higher value at around 9%. This corresponds to the 

theoretical volume flow increase. In fact, the measured volume flow deviates from 

the reference volume flow by + 28%. The additional measuring error is a systematic 

error associated with the strongly disturbed flow around the vane and the small 

depth of penetration into the duct. 

D=250 

INLET NOZZLE 

Measured Values (m/s) and Results of Meas. Series 

with a Prandtl Pitot Tube (Comparative Measurement) 

Reference Volume Flow 1512 m 3 

/h 

Dimension 

8 

26 

49 

81 

169 

202 

224 

242 

y x 

8.7 

9.1 

9.3 

9.4 

9.5 

9.7 

9.3 

8.9 

8.3 

9.2 

9.3 

9.6 

9.7 

9.7 

9.4 

8.8 

Mean velocity 

9,2 m/s 

Duct section 

0,0491 m 2 

Volume flow 

1633 m 3 

/h 



+8 %

• Backflow effects 

When using vane probes, the occurrence of backflow is not evident from the measured 

values, because all the values displayed are positive figures (the direction of 

rotation of the vane cannot be determined from the subsequent measuring technique). 

This interrelationship is also apparent from the comparative measurement 

illustrated in Fig. 31. 

• 

If severe backflows occur due to intrusions into the duct that are not visible from the 

outside (e.g. sound absorbers in a thermally insulated duct or objects left in the duct 

following assembly), too high air flows are measured as a rule. 

Recommendation: In such cases, directional display probes should be used, such 

as thermal probe 0635.1042 or Pitot tubes in conjunction with 

pressure probes. 

Increase in the Horizontal Velocity Profile with the 

Electronic Vane Anemometer (dia. 16 mm) 

A 

10 m/s 

5 m/s 

A 

1 D 

x = 0 

SECTION A-A 

D = 250 

AIR 

x (mm) y (m/s) 

8 3,9 

26 4,4 

49 4,6 

81 5,7 

169 8,5 

202 9,4 

224 10,5 

242 10,6 

Increase in the Horizontal Velocity Profile with the 

Prandtl Pitot Tube 

A 

Fig. 31: Comparison of the horizontal velocity profile measured with the vane 

anemometer (dia. 16 mm) and the Prandtl Pitot tube immediately after 

the bend. The backflow (swirl area, no defined direction of flow) is not 

picked up by the vane anemometer. 

10 m/s 

5 m/s 

A 

1 D 

x = 0 

SECTION A-A 

D = 250 

AIR 

x (mm) y (m/s) 

8 (-3,2) 

26 1,3 

49 7,0 

81 8,3 

169 10,1 

202 10,6 

224 11,5 

242 11,0 

43

Hints on Using Pitot Tubes 

• Using Pitot tubes carefully 

The Prandtl probe has two pressure-measuring points (static pressure and dynamic 

pressure). Plastic hoses are used to connect it to the differential pressure sensor. 

Care must be taken to ensure that the plastic hoses fit the connecting branches and 

any connectors used snugly. Otherwise, the difference in pressure with the 

environment will create disturbances in the probe and hose. 

The resulting pressure drop can severely distort the measurement signal. Errors of 

this type can also occur if the connecting hoses are damaged. Bearing this in mind, 

the hoses should be handled with care and checked for tightness before each measurement. 

Measuring errors are also frequently caused by the hoses being inadvertently 

compressed or bent. 

• Contamination 

If measurements are made of kitchen or industrial exhaust air, the dynamic pressure 

measurement aperture may be altered by the particles present in the air. It should be 

regularly visually inspected and cleaned. 

• Incorrect evaluation of measurement results 

The mean flow velocity in the duct section under test is calculated from the 

arithmetic mean of the individual velocities. This in turn is calculated from 

the dynamic pressure measured, as follows: 

44 

v = flow velocity in m/s 

ρ = air density in g/m 3 

p = differential pressure measured at the Pitot tube in hPa 

• Frequently, evaluation takes the form of calculating the mean from the dynamic 

pressures measured and then calculating the mean flow velocity as shown above. 

This method is mathematically incorrect and leads to unacceptable deviations 

from the correctly determined value, especially with distorted flow profiles.

• Another error commonly arises from calculating with an average density of 

1200 g/m 3 

. When measuring external air flows, the actual air density can d 

eviate from this average by as much as ± 10%. This leads to an air flow uncertainty 

of up to ± 5%. 

This is where you can make use of the properties of the testo 452 or 454: by activating 

the automatic conversion of the dynamic pressure into flow velocity, the mean 

can be calculated directly from m/s values. The only thing to remember is that the 

correct air density must be entered in the configuration menu. This may be determined 

from tables, provided the local values for absolute pressure, temperature and 

if relevant the relevant humidity are known. 

It is advisable to record this air density value and the parameters used in calculating 

that value (temperature, absolute air pressure and humidity) in the log. 

Pitot tubes are the ideal sensors for flow measurement in the medium to high 

velocity range, at high temperatures or in polluted air. Below 5 m/s they have only 

a limited application, because the differential pressure is associated with too great 

errors at the lower end of the range. 

A usable pressure measurement can only be obtained with sensitive instruments 

and the greatest care. Instead, measurement with thermal or vane sensors is 

recommended below 5 m/s. 

Air flow measurement at vents 

As already described in the section on "Measuring errors in air flow measurement" 

(page 36), air flow measurements should not be made at vents, because the method 

used in practice is both very time-consuming and carries a high risk of error. 

The two frequently used methods, calculation of the mean and the loop method, 

are nevertheless described below. 

45

46 

LLaammiinnaarr ffllooww iinn tthhee 

cceennttrree ooff tthhee dduucctt 

Max. max. Werte value 

Min. min. Werte value 

Mean gemittelte values Werte 

Fig. 32: 

Integral measurement 

with a large vane or 

calculation of the 

mean with a dia. 16 mm 

vane probe. 

The air vent greatly changes the relatively uniform flow inside the duct: areas of higher 

flow velocity are created at the free vent surfaces and areas of low flow velocity and 

swirl at the grilles. The flow profile steadies at a distance from the grille which 

depends on the grille design (approximately 20 cm). A flow profile which swells and 

declines at intervals remains. 

Fig. 33: 

Use of the testo 452 

Usable measurements generally require large vane diameters in this case 

(60 or 100 mm diameter), because with such large diameters the flow values 

are integrated and the mean calculated over a larger area. 

If small-diameter vanes are used (e.g. 16 mm), the mean has to be calculated at 

intervals with a corresponding number of measuring points.

When large vanes are used, looped examination of the grille section (in conjunction 

with mean calculation over time) is sufficient. 

Measurement 

at intervals 

Mean value 

over time 

Fig. 34: 

Grille measurement 

at the duct exit 

Once the mean air velocity has been determined, the volume flow may be calculated 

by multiplying this value by the clear grille flow section. 

Example: V = 1 m/s x 0.8 x 0.2 m x 0.5 m 

where: 1 m/s measured velocity, 0.2 m x 0.5 m grille area; 

0.8 : 20 % of the grille area covered by fins. 

The following sources of error affect the measurement result: 

• Irregular vane guide speed 

To allow for this, several measurements should be made, e.g. once with the probe 

guided in vertical loops and once in horizontal loops. The mean obtained should 

be used in further calculations. 

• Air flow affected by the vane and the person making the measurement 

Avoid blocking the air exit unnecessarily, because any resistance to the flow will 

affect the measurement result. If possible, use a large vane with a telescope so 

that only the vane probe is in the flow profile to be measured. 

47

• Irregular grille exposure 

The exposure of the grille section depends on the air velocity in the duct upstream 

of the air vent. Fig. 35 shows an air duct with 3 grilles. The flow profile at the first 

grille is very irregular, because the greatest velocity of flow occurs in the duct 

upstream of this vent. 

In the right-hand part of the grille there is a backflow, i.e. ambient air is sucked into 

the duct. Moreover, the direction in which the fresh air is blown out deviates sharply 

from the vertical. 

48 

Grille 3 Grille 2 

Air duct without section reduction 

Grille 1 

Fig. 35: Schematic View (Simplified) of the Velocity Profile at the Air Vent to 

Illustrate the Irregular Exposure of the Grille Section 

• The distance from the vane to the air vent cannot be defined 

The air velocity increases between the vanes. Directly downstream, areas of swirl 

with no defined direction of flow occur, irrespective of the angle of deflection. The 

use of vanes is confined to directed flows with no marked velocity peaks. 

The vane should therefore be guided across the grille surface at a certain minimum 

distance (guide value 5 cm). As the distance from the air vent increases, the velocity 

profile does admittedly even out, but on the other hand the jet of air broadens as the 

distance increases and the mean velocity decreases. It is difficult to determine the 

section area to be used in the volume flow calculation - see below.

• The clear flow section to calculate the volume flow cannot be defined 

The area corresponding to the measured velocity can only be estimated. In the case 

of air grilles, the clear flow section is taken as a starting point. Determining the area 

in this way is only possible with straight lines (no deflection). In all other cases, the 

air flows are too high. The same applies to multi-part grilles (e.g. with quantity 

adjustment). 

In many cases the reduction in section due to the grille fittings is estimated after 

measurement in order to reach approximately the desired air flow. 

Summary 

Measurement of mean air velocity at air vents by the loop method only yields verifiable 

(repeatable) results if the entire grille section is evenly exposed and the vanes are 

straight (without deflection). 

Such a case only rarely occurs in practice, and the loop method is therefore of only 

limited use for acceptance measurements and should only be used for estimated 

measurements. This applies analogously to calculation of the mean at intervals 

using small-section probes. 

Measurement at air vents can be improved by using a high-resistance foam duct 

attachment or clamping plates, as shown in Fig. 36. At the end of the duct a grid 

measurement is made by the trivial method, e.g. using a Prandtl Pitot tube.The 

length of the duct must be at least 4 times the hydraulic diameter 

(4 D h; D h = 4 A/U, calculated from the grille section). 

With an irrotational flow and low pressure drop due to the duct attachment, this 

method yields sufficiently accurate results. In theory, however, the air flow measured 

is always too small. 

49

50 

Duct attachment 

Seal Measuring plane 

Fig. 36: 

Duct attachment with 

grid measurement 

(from the Manual of Air 

Conditioning Engineering, 

Volume 3 [5]) 

A tapered duct attachment is often used for adjustment work on smaller air vents, a 

vane being placed at its centre in the constrained flow. In this case the absolute value 

of the air flow is not significant, because all that is required is to balance the individual 

air vents. The total air flow in the duct is determined by grid measurements. 

The volume flow at air vents can be accurately measured by the zero method. 

Because of the large amount of apparatus required, however, this method is only 

exceptionally used in practice. Other methods (e.g. the airbag method, which 

are also cumbersome, however) are described in the Manual of Air Conditioning 

Engineering [8]. 

Measurements at Suction Apertures 

As already explained, measurement at outlet grilles (blowers) is critical, but nevertheless 

feasible within certain limits. While the flow profile is certainly influenced by the 

grille, it is still maintained even at some distance for the grille as a rule, and the 

velocity can therefore be measured. 

The conditions are different at apertures which extract air from the room. Even 

without the disturbing effects of a grille, the lines of flow are not directional and the 

flow profile is highly unhomogeneous.

Suction 

- Measurement using 

a funnel 

Blowing 

- Measurement using a 

large vane 

- Calculation of the mean 

at points/intervals 

Velocity probe 

Fig. 37: Lines of a flow at Fig. 38: Determining the volume flow 

suction apertures with a measuring funnel 

The reason is that the partial vacuum in the duct draws air out of the room in a funnel 

shape. At even a short distance from the aperture there is no defined area in the 

room over which a measurement could be made, for example a grid measurement 

with calculation of the mean, to determine the volume flow. Only the duct measurement 

yields reproducible results. 

Disk valves are a special case. The volume flow drawn off is adjusted by rotation of 

the valve. Here too, the clear flow section cannot be determined, being at the core of 

the valve (an annulus); nor can a velocity probe be placed at this point. 

Measuring funnels of various sizes are available for such applications. These create 

defined flow conditions at a distance from the disk valve with a fixed section. The 

velocity probe is positioned centrally here and fixed. 

The volume flow drawn off is calculated from the measured value of the velocity probe 

multiplied by the funnel factor (e.g. funnel factor 20). 

51

Determining volume flow by the fan characteristic 

Determining the volume flow at the central unit of the air conditioning plant by means 

of a differential pressure and speed measurement is permissible only if the equipment 

characteristic is available. This can either be taken on the manufacturer's test bench 

or in situ by calibration (see also draft DIN 4796 [7]). 

If the volume flow is taken from the fan characteristic (manufacturer's reference 

material), considerable deviations from the actual volume flow can occur depending 

on where the fan is fitted (see illustration in Fig. 39). 

The fan characteristic is determined on a standardised test bench. The upstream and 

downstream conditions may vary considerably from those at the central unit. 

52 

Differential pressure 

Meas. 

value 

➀ Conditions in situ 

➁ Theoretical fan characteristic 

1 

2 

Error 

Correct value Volume flow 

Fig. 39: 

Errors of interpretation 

when determining 

volume flow from the 

fan characteristic

Measuring the Ambient Air Velocity 

Ambient air velocity is a very important parameter in connection with thermal comfort 

of people in common rooms. 

Mean air velocity 

0,5 

0,45 

0,4 

0,35 

0,3 

0,25 

0,2 

0,15 

0,1 

0,05 

0 

20 

Degree of turbulence 

21 22 23 24 25 26 27 ° C 

Preparing for and Carrying out Measurements 

Fig. 40: 

Mean air velocities as 

a function of temperature 

and degree of air 

turbulence in the 

comfort range 

In principle, measurements should be made in the room with all furniture and equipment 

in place, because furniture and items of equipment have a considerable influence 

on the flow in the room. 

Before starting to make measurements, the marginal conditions must be established 

and adjusted accordingly. In particular, the setting of the air vents and the difference 

between the fresh air and ambient air temperature should be checked, as these have 

the greatest influence on the air distribution and ambient air velocity in the common 

area. 

5 % 

20 % 

40 % 


53

It must also be ensured that an undesired exchange of air is not taking place at the 

surfaces enclosing the room (windows and doors), as this can cause draught phenomena 

not attributable to the air conditioning system. 

Measuring points are normally selected only with the area used by people. Here 

the air flow from the adjacent air vent can be made visible by means of test smoke 

(N.B.: flow velocity probes should not be exposed to smoke!). 

If draughts are detected in the area of the ankles, a check should be made whether 

this is due to cold air downflow at windows. The mean ambient air velocity, (scalar) 

degree of turbulence and air temperature should then be determined at the critical 

points thus identified, preferably at heights of 0.1 m (ankle area), 1.3 m (head height 

when seated) and 1.8 m (head height when standing). 

Pressure Measurement 

The difference in static pressures is measured as part of acceptance measurements 

to check the pressure drop caused by fittings (such as air filters). As Fig. 41 shows, 

the following points should be borne in mind when making measurement holes: 

54 

• No dynamic flow components should be allowed to affect the pressure measuring 

point. The flow must be parallel to the wall. 

• The measurement holes should be as small as possible (D= 1-2 mm) and 

must be burr-free. The pressure measuring connections should be attached 

to the outside of the duct, centred on the hole, to form a tight seal.

WRONG WRONG RIGHT 

Air flow Air flow Air flow 

Duct 

Measuring hose 

Burr Ø 1to 2 mm 

Duct 

Pressure-measuring 

connection 


Fig. 41: Errors in making pressure measuring points on air ducts. 

The illustration on the right shows the correct way. 

When measuring static pressure against ambient pressure, the differences in height 

between the static pressure measuring point and the ambient pressure measuring 

point should be corrected on the measuring instrument. This problem can be avoided 

by having the two pressure measuring points (open inlet to the instrument and static 

pressure measuring point on the duct) at the same height. 

CO2 Measurement as a Means 

of Assessing Room Air Conditions 

In the German-speaking countries of Europe this is still in its infancy, whereas in 

Scandinavia, the US and Japan it is already firmly anchored in the consciousness of 

air conditioning and ventilating technicians and engineers and the subject of much 

discussion.The topic is CO 2 measurement as a basis for needs-oriented ventilation 

control. 

Why should a parameter virtually disregarded until now be introduced into everyday 

air conditioning and ventilation measuring engineering as a new parameter and that 

parameter made an established component of acceptance measurements along with 

temperature, humidity, air velocity and pressure? 

In large buildings and office complexes in particular, it is no longer possible today to 

adjust the fresh air supply quickly by opening windows when required. The job an air 

conditioning plant is required to do is precisely to deliver the optimum atmospheric 

conditions to the right room at the right time, having regard to an optimum sensation 

of comfort for the people spending time and working in the room. 

Duct 

Pressure-measuring 

connection 


55

And of course it must also operate in an energy-efficient way. 

How does this look in practice? 

On the one hand, saving energy may be the primary consideration, leading to an air 

conditioning plant oriented to a minimum exchange of air in the individual rooms and 

operating with a high proportion of ambient air. Here, naturally, draughts in the rooms 

are kept to a minimum. 

However, bottlenecks build up in the "fresh" air supply, particularly when more people 

than average are in the ventilated rooms. General discomfort sets in, accompanied by 

loss of concentration, lassitude and a drop in efficiency. All signs of the much-talkedabout 

"sick building syndrome". 

The other alternative is for the ventilation plant to be adjusted for the maximum number 

of people who can be in the rooms. In this case, while sufficient fresh air is certainly 

always available, the general sensation is an unpleasant one, and it is felt to be 

"draughty". A further effect of this is that more energy is consumed on average than 

the situation requires. 

The measured CO 2 value is in fact the ideal indicator of ambient air consumption, showing 

whether the proportion of fresh air in the air supply needs to be increased or may 

be decreased. In other words, the CO 2 content is regarded as the parameter that enables 

a direct decision to be taken on air quality. 

Unpolluted fresh air has a CO 2 content of around 350 ppm. If a person in a sedentary 

occupation is assumed to emit approximately 20 l/h CO 2, there is a high CO 2 concentration 

in a closed room despite the supply of external air. 

56

External Air Supply per Person CO2 Concentration 

[m 3 /h/person] [ppm] 

3,8 5000 

8,5 2500 

14,9 1500 

25,6 1000 

Table 3: 

Steady-state value of 

CO 2 concentration in 

a room as a function 

of external air supply 

Control engineering already takes these conditions into account, as more and more 

plants are being fitted with CO 2 measuring transducers to adjust the air supply to the 

conditions required at any one time. 

Although the limits to be observed have not yet been laid down by law for the various 

applications, there are nevertheless established experience values, which vary from 

1000 to 7000 ppm depending on the application. 

CO2 probes in conjunction with a data logger offer the best solution for adjustment 

work on air conditioning plants and control measurements in rooms with a constantly 

changing number of people. Random measurements can be carried out just as satisfactorily 

as observation over longer periods. See testo 454 with CO 2 probe. 

General Hints on Using Measuring Instruments 

• Measuring instruments should be checked and recalibrated if necessary well in 

advance of use. In the case of measurements under ISO 9000 in particular, the 

measuring instruments have to be recalibrated at specified intervals. Testo offers 

calibration certificates for the complete measuring system (i.e. display unit and 

probes for ISO 9000 certification). Testo is an approved DKD laboratory for 

temperature and humidity. 

• Enough spare batteries should be carried when making measurements. Storage 

batteries should be fully charged and have enough available storage capacity. 

57

• When making the measurement, make sure that the ambient temperature lies within 

the range specified by Testo. When measuring instruments are carried about in cold 

winter weather (e.g. in a car boot), enough time must be allowed for the instruments 

to come up to their operating temperature. Measuring instruments should be left in 

the case while warming up to avoid condensation. 

• If the measured value displayed fluctuates greatly, it is advisable to calculate a 

mean, either by taking the mean of the displayed values over a period of time or by 

calculating the mean of a large number of different measuring points. The individual 

values used in calculating the mean should also be logged. This is essential for a 

clear understanding of the measurement results in subsequent checks. 

Handling Probes and Sensors 

• Air flow measurement probes should be compared with calibrated reference probes 

from time to time. The reference probes are not exposed to the wear and tear of 

everyday use and thus remain accurate over long periods. 

Neither a wind tunnel nor reference probes are needed for a simple routine check. 

Vanes need only be blown on gently. The vane will turn absolutely evenly with no 

incidental noise (slight rattle). A vane in perfect condition will turn evenly until it comes 

to a stop. If the vane wobbles before coming to a stop, this indicates that individual 

fins are warped. If vanes do not turn when blown on gently, but tend to run backwards 

when started up at higher flow velocities, the bearings are dirty and should be cleaned 

in accordance with the manufacturer's instructions. 

For thermal velocity probes, visual inspection is generally sufficient. 

Comparative measurements of different types of velocity probes, e.g. comparisons 

between a thermal probe and a vane, should only be made under ideal flow conditions. 

The flow conditions should be as close as possible to calibration conditions, 

i.e. measurement in laminar flow, measurement in the free jet (350 mm), 

not in the duct. 

Accurate comparative measurements take a great deal of time and require a lot of 

equipment (wind tunnel and reference system). In practice, the ambitious accuracies 

guaranteed in the reference material play only a subordinate role. In 

58

everyday use, the accuracy and meaningfulness of the measurement results is 

determined almost exclusively by the correct use of the measuring instruments 

and the choice of the right measuring point. 

• Visual inspection suffices for surface temperature probes. 

The band probe or pipe surface probe 

(with rolled sprung thermocouple bands) 

should be inspected for probe breakage 

from time to time. 

Fig. 42: 

Band Probe 0600.0194 

The accuracy of temperature probes for air and immersion measurements is not 

critical provided they are adjusted as specified by Testo. 

• Humidity probes are maintenance-free as a rule. An inspection and calibration set 

is available for occasional checking and adjustment. 

• Differential pressure probes should be calibrated at zero before each measurement 

(see instrument description). To avoid overload, the measurement should be started 

with a probe with a high measuring range. Precision probes with a small measuring 

range can then be used for more accurate measurements at a known pressure. 

59

Presenting Testo Measuring Instruments 

Testo portable hand-held measuring 

instruments come in three 

categories: 

1)Compact test instruments for 

precise point measurements. 

The measurement data are 

not documented. Ask for 

our measuring instrument 

catalogue. 

2)With professional system 

instruments, documenting the 

measured values is the main 

consideration: storage and 

printing out of data and processing 

using your PC. 

Tedious writing-up is a thing of 

the past, and reading and 

transfer errors are eliminated. 

Send for detailed information 

or contact us to arrange a 

demonstration. 

3)Measured data storage units 

automatically measure and 

record measured data over 

long periods. Data loggers of 

this type are used to monitor 

air conditioning and refrigera 

tion plants, to make series of 

measurements for instance on 

test benches and as part of 

quality assurance in produc 

tion, storage and transport. 

The data stored are processed 

using the PC. Send for detailed 

information or contact us to 

arrange a demonstration. 

60 

Measuring 

Instrument 

Measuring 

Infrared 

Printer 

Printing out 

PC- 

Software 

Processing 

measured data 

Recorder 

Printing out 

and storing

Our consistent system approach offers decisive advantages: 

• The recorder simply plugs in to turn the Testo measuring instrument into a handy, 

intelligent measuring system. 

The measured data can be stored and printed out on location. 

Cableless infrared printing is also possible. No more time-consuming writing-up, 

and reading and transfer errors are eliminated. 

• Testo Comfort software communicates with all the equipment in the system and 

evaluates the measured data. 

• Probes and accessories fit all system equipment. 

A carefully designed system that brings benefits to the user. 

Calibration certificates 

Testo offers calibration certificates for the following parameters: temperature, humidity, 

velocity, pressure, flue gas, luminous intensity and rpm. 

As a rule, standard calibration certificates 

with fixed measuring points and special 

calibration certificates with selectable 

measuring points are available for each 

parameter. Testo is also an approved 

calibration laboratory for temperature, 

temperature of dew point and relative 

humidity and is thus able to issue DKD 

certificates for measuring instruments 

and probes. 

Please ask for detailed information. 

61

Summary of Instrument Data 

System instruments 

testo 700 / 701 

Pt 10 

-200 to +800°C 

testo 781 

Quar. 

(Resolution 0.01°C) 

-40 to +300°C 

testo 900 / 901 

Thermocouple 

-40 to +1370°C 

testo 9500 

Thermocouple/ 

NTC 

-200 to +1760°C 

testo 9600 

Thermocouple / 

NTC 

Explosionproof and 

firedampproof, appropriate 

for verification 

-200 to +1370°C 

Data loggers 

testostor 171 

NTC 

-50 to +120°C 

62 

Temperature 

Humidity 


testo 600 / 601 

0 to 100%RH 

-20 to +140°C 

with dew point 

determination 

Data loggers 

testostor 171 

0 to 100%RH 

-50 to +120°C 

Velocity 


testo 490 / 491 

0 to 60 m/s 

-40 to +350°C 

Vane probes 

Thermal probes 

Combined 

Measuring 

Instruments 


testo 451 

-120 to +1370°C 

0 to 100%RH 

0.2 to 60 m/s 

testo 452 

-120 to +1370°C 

0 to 100%RH 

0 to 100 m/s 

±100 hPa 

(Differential pressure) 

testo 454 

Measuring instrument 

and data logger 

-200 to +1370°C 

0 to 100%RH 

0 to 60 m/s 

±100 hPa (Diff.pressure) 

0 to 2000 hPa(Absolute 

pressure) 

0 to 1 Vol% CO2 30 Hz to 300 kHz 

0 to 20 mA 

-10 to +10 mV 

100 W to 300 kW

Surface Probes 

1) Rugged Pt 100 probe with a broad measuring tip 

formeasurements on flat surfaces 

testoterm 

testoterm 

2) Rugged NiCr-Ni probe with a broad measuring tip for 

measurements on flat surfaces 

3) High-temperature surface probe with sprung 

thermocouple band 

testoterm 

4) Velcro probe for attachment to pipes etc. for long-term 

measurements 

5) Magnetic probe for high temperatures 

a) 

a) Adhesion approx. 10 N, b) Adhesion approx. 20 N 

b) 

6) Clamp-on pipe probe for temperature measurement on 

pipes up to 2" diameter 

7) Infrared probe for contactless temperature measurement 

on live, inaccessible and rotating parts 

Measuring 5 7.5 14 21 33 mm 

spot diameter 

Measuring 16 25 50 76 130 mm 

distance 

testo 

Measuring 

Range 

-50 to +400 °C 

-200 to +600 °C 

-200 to +700 °C 

-50 to +150 °C 

-50 to +400 °C 

-50 to +170 °C 

-60 to +130 °C 

-18 to +260 °C 

Sensor 

Pt100 

(Class B) 

NiCr-Ni 

NiCr-Ni 

(Class 2) 

Pt100 

NiCr-Ni 

NiCr-Ni 

(Class 2) 

NiCr-Ni 

(Class 2) 

Infraredsensor 

t 99 

(sec) 

40 

25 

3 

40 

– 

– 

5 

2 

Probe Meas. 

tube length tip diam. 

150 mm 

150 mm 

200 mm 

130 mm 

Probe 

length 

180 mm 

8 mm 

4 mm 

8 mm 

25 mm 

25 mm 

65 mm 

– 

63

Immersion Probe 

8) Probe for measurement in liquids and powdery 

substances. 

testoterm 

Penetration Probe 

9) Watertight probe with ground measuring tip, boilproof 

Air Probes 

10) Ultra-fast response probe for measurement in liquids 

and gases 

testoterm 

11) High-precision probe for air / gas temperature 

measurements 

testoterm 

12) Globe thermometer for radiant heat measurement 

Thermocouples and Adapters 

13) Stick-on thermocouple for surface 

measurementsCarrier material: aluminium foil 

14) Adapter to connect thermocouples and probes 

with bare wire ends 

a) 

b) 

Measuring 

Range 

-200 to +600 °C 

-200 to +600 °C 

Measuring 

Range 

-200 to +600 °C 

Measuring 

Range 

Max. temp. 

+200 °C 

Sensor 

-200 to +600 °C NiCr-Ni 

-40 to +130 °C 

-25 to 80 °C 

Sensor 

NiCr-Ni 

Pt100 

Sensor 

Pt100 

Dimensions 

Thickness 

0.1 mm 

ø Extension 

2.0 x 0.2 mm 

Attachment to 

measuring 

point 

With standard 

adhesives or silicone 

heat paste, 

part no. 

0554.0004 

Supplied 

as 

Pack 

of 2 

All Pt100 data to DIN IEC 751, Class A. 

Thermocouple technical data to DIN IEC 584 Part 2, Class 1. 

64 Temperature probes with PtRh-Pt or FeCu-Ni loggers on request. 

NTC 

t 99 

(sec) 

1 

20 

t 99 

(sec) 

30 

t 99 

(sec) 

9 

60 

Probe tube 

length 

150 mm 

150 mm 

Probe tube 

length 

150 mm 

Probe tube 

length 

150 mm 

150 mm 

P.t.diameter 

1.5 mm 

3 mm 

M.t.diameter 

3 mm 

M.t.diameter 

0.5 mm 

9 mm 

ø Ball 

approx. 150 mm

Humidity Probes 

15) Air probe for humidity and temperature measurements 

16) High-temperature humidity probe for measurement 

e.g. in ducts or bulk material 

testoterm 

17) High-temperature humidity 

probe for measurement 

e.g. in ducts or bulk 

material 

testo 

Pressure Probes 

testo 

term 

18) Pressure-resistant humidity probe for measurement in 

compressed air plants with standard plug 

19) Differential pressure probes a) and b) for 

measurement of flow velocity (in conjunction with Pitot 

tube) and measurement of differential pressure 

0638.1445 

Ø Pitot tube connection dia. 5 mm 

Absolute pressure probe c) for absolute pressure measurement 

20) Pitot tubes, various lengths, diameters and materials, 

for measurement of flow velocity (in conjunction 

with differential pressure probes) 

testo 

Measuring 

range 

a) ±100 hPa Diff.-pressure 

b) 

c) 

Measuring 

range 

0 to 100 %RH capacitive 

(Probe tip) NTC 

-20 to +70 °C 

0 to100 %RH capacitive 

-20 to +140 °C NTC 

0 to 100 %RH capacitive 

-20 to +140 °C NTC 

0 to 100 %RH 

Pressure dew 

point t pd: 

-50 to +40 °C 

±10 hPa 

2 bar 

Material 

a) Brass 

b) Brass 

c) Stainless 

Steel 

Sensor 

capacitive 

NTC 

Measuring 

system 

Diff.-pressure 

Abs. pressure 

Temp.max 

350 °C 

350 °C 

500 °C 

t 90 

(sec) 

10 

20 

20 

Length 

Overall 

length 

245 mm 

Probe 

tube 

300 mm 

Probe 

tube 

1500 

mm 

Accuracy 

Ø 

21 mm 

Probe 

tube 

12 mm 

Tip 

12 mm 

Overall length 300 

mm 

Standard plug 

connection 

System accuracy for 15) - 18): ±2 %RH (2 to 98 %RH) ±0.4 °C (0 to50 °C) ±0.5 °C (residual range) 

± 0.1 hPa (0 to 20 

hPa) 

± 0.5 % o. vm. (20 to 100 

hPa) 

± 0.03 hPa 

± 5 hPa 

Length Ø 

500 mm 7 mm 

350 mm 7 mm 

300 mm 4 mm 

65

Vane Probes 

(System accuracy with instrument) 

21) Combined vane/temperature probe 

(plug-in)*, Ø 25 mm 

22) Combined vane/temperature probe 

(plug-in)*, Ø 16 mm 

23) Low-temperature anemometer probe with handle, 

operating range -20…+60 °C 

rm 

24) High-temperature probe for long-term 

measurements up to +350 °C, 

Short-term measurements up to +500 °C 

25) Shell anemometer (not shown) 

26) Vane Ø 60 mm for measurement at the duct outlet, 

with pull-out telescope, operating range 

-20 to +60°C 

27) Vane Ø 100 mm for measurement at grille outlets, 0.2 to15 m/s Vane ±0.3 m/s 280 mm 100 

with handle (not shown) 

mm 

* Plug-in probes also require a handle or telescope (see ordering details) 

** For short-term measurements 

testoterm 

Thermal Velocity Probes 

28) Value-for-money rugged probe for measurements at 

the lower end of the velocity range 

66 

testoterm 

29) Rugged probe with telescope for measurements at 

the lower end of the velocity range 

30) Fast-response probe with telescope for measurements 

at the lower end of the velocity range, with flow direction 

detection 

Measuring 

range 

0.4 to 40.0 m/s 

-30 to +140 

°C** 

0.4 to 60.0 m/s 

-30 to +140 

°C** 

0.6 to 40.0 m/s 

0.4 to 20.0 m/s 

-40 to +350 °C 

0.5 to 35 m/s 

0.25 to 20.0 m/s 

Measuring 

range 

0 to10.00 m/s 

-20 to +70 °C 

0 to 10.00 m/s 

-20 to +70 °C 

0 to 10.00 

m/s 

0 to +50 °C 

Sensor 

Vane ±1 % 

NiCr-Ni of final value 

Vane 

NiCr-Ni 

Vane 

Vane 

NiCr-Ni 

Vane 

Sensor 

Hot ball 

NTC 

Hot ball 

NTC 

Hot wire 

NTC 

Accuracy 

±0.4 m/s 

(up to 40 

m/s) 

±2 % 

of final value 

±2.5 % 

of final value 

±0.3 ms 

±5 % o. mv. 

±0.2 m/s 

±2 % o. 

mv. 

Probe 

length 

150 mm 

190 up to 

850 mm 

160 up to 

760 mm 

Probe 

length 

180 

mm 

180 

mm 

190 

mm 

560 

mm 

440 

to 

1100 

mm 

Tip 

dia. 

25 

mm 

16 

mm 

16 

mm 

25 

mm 

60 

mm 

Ø 

Tip dia. 

4 mm 

4 mm 

Hot ball accuracy: ±0.05 m/s, ±2.5 % of m.v.(0 to 2 m/s) ±0.5 m/s, ±5 % of measured value (2 to 10 m/s) 

Temperature compensation: < 0.2% of measured value/°C (-10 to+60 °C)

Sources and Bibliography 

68 

[1] 

VDI 2080: Measuring Methods and Measuring Instruments for Air 

Conditioning Plants 

October 1984 

[2] 

VDI 2079: Acceptance Testing of Air Conditioning Plants 

March 1983 

[3] 

DIN 1946 Teil 2: Air Conditioning Engineering, Health Requirements 

January 1994 

[4] 

Glück, B: Air Conditioning Engineering, Health Requirements 

GI Domestic Engineering, Structural Physics, Environmental 

Engineering 1993, Volume 3 

[5] 

Air Conditioning Engineering Lecturers' Working Party: Manual of 

Ambient Air Measurement, Volume 3 

[6] 

DIN 33403, Part 1: Air Conditions at Work and in the Working Environment, 

April 1984. 

[7] 

DIN 4796 Part 1 and Part 2: Measuring the Efficiency of Air Conditioning Plant 

Draft, September 1991 

[8] 

Air Conditioning Engineering Lecturers' Working Party: Manual of 

Ambient Air Measurement 

Volume 1: Fundamentals

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Ambient Air Measurement for Practical Users

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